Hot Topics:

CU study ties drought to spruce beetles

Outbreaks also historically coincide with changes in North Atlantic surface temps

By Charlie Brennan Camera Staff Writer

Posted:
10/13/2013 11:29:21 PM MDT

Updated:
10/13/2013 11:30:03 PM MDT

Drought in the high northern Colorado mountains, tied to long-term changes in ocean-surface temperatures in the Northern Atlantic Ocean, has been linked to a massive spruce beetle outbreak, according to researchers at the University of Colorado.

"It is simply a wakeup call that shows that alarming trends are resulting in major changes in ecological disturbances, that are probably having significant changes on vegetation structure, vegetation patterns today," said CU geography professor Thomas Veblen, a co-author on the new study published online in the journal Ecology.

Pine beetle infestation has received more public notice in Colorado in recent years, but spruce beetles, closely related, are also affecting increasingly large expanses of coniferous forests across the western United States and have the potential to be more destructive in Colorado, Hart said.

The most recent U.S. Forest Service and Colorado State Forest Service annual aerial forest health survey showed that the mountain pine beetle infestation has slowed dramatically, while the spruce beetle outbreak is expanding.

The survey showed that the mountain pine beetle epidemic expanded by 31,000 acres, which is down from last year's expansion of 140,000 acres -- which brought the total pine beetle infestation to 3.4 million acres in Colorado since 1996.

The spruce beetle infestation is growing much faster, however, according to the survey, with a one-year jump of 183,000 new acres in 2012, bringing the total affected in the state to 924,000 acres since 1996, according to the federal and state forest services.

Advertisement

Veblen put the number higher, at 960,000 acres.

"People are well aware of the mountain pine beetle outbreak partly because they affect the lodgepole pines and they see them as they drive along I-70," said Veblen. "The spruce pine beetle infestation occurs at higher elevations. You have to get really back onto the backroads, high up, to see it."

He cited the Rio Grande and San Juan National Forests in southern Colorado as being particularly affected.

Sarah Hart, a doctoral student in geography at CU and lead author of the recently published study could not be reached for comment Friday. But, in a news release, she said, "It was interesting that drought was a better predictor for spruce beetle outbreaks than temperature..

The CU team assembled a long-term record of spruce beetle outbreaks from the northern Front Range to the Grand Mesa in western Colorado using a combination of historical documents and tree ring data from 1650 to 2011. Broad-scale outbreaks were charted by the team from 1843-1860, 1882-1889, 1931-1957 and 2004 to 2010.

The CU study area included historical data from sites in the White River, Routt, Arapaho, Roosevelt and Grand Mesa national forests, as well as in Rocky Mountain National Park.

The scientists used a variety of statistical methods to distinguish between so-called warming signals and drought signals for 18 sites in Colorado. Several lines of evidence emerged showing drought to be the main driver of the spruce beetle outbreak.

The strongest climate correlation to spruce beetle outbreaks was above-average annual values for the Atlantic Multi-decadal Oscillation, a long-term phenomenon that changes sea-surface temperatures in the North Atlantic. Believed to shift from cool to warm phases about every 60 years, positive AMO conditions are linked to warmer and drier conditions over much of North America, including the West.

The current AMO is believed to be only in the first 10 to 15 years of its cycle, so the conditions that have historically coincided with regional drought -- and the resulting infestations -- may continue for several decades.

"That's really the new finding," Veblen said. "From a historical perspective, when you look back over the last few thousand years, it's not just the warming. It's the drought that predisposes the trees to attack. We're saying it's the combination of the two. It's both the warmer temperatures and the drier conditions.

"The effect of drought, itself, has been understated."

Other co-authors on the study include former CU graduate student Karen Eisenhart, now at Edinboro University of Pennsylvania; and former students Daniel Jarvis and Dominik Kulakowski, now at Clark University in Worcester, Mass. The study was funded by the National Science Foundation .

New coordinator pushes Buffs to work, play at level he expectsJim Leavitt has discovered this much about his new defense at Colorado: He has some talent with which to work, but his players need to put it in another gear. Full Story